Part drawings almost always provide dimensions. What’s less common is any specification of the surface. On the occasions it is given it usually takes the form of an Ra, or less often, Rz, value, but often it’s overlooked entirely.
That’s a mistake, possibly an expensive one. Surface quality has a major influence on how a component or assembly performs in service. Keep reading to learn about six of the most important reasons for quantifying the surface needed. First though, a short discussion of what’s meant by surface quality.
The characteristics of a surface
When producing metal parts, machined or fabricated, it’s usually the surface roughness that’s of greatest interest. However, “roughness” is an imprecise term. There’s the macro form, which may show waviness, and the micro surface, where features like pits and crevices become evident.
A second characteristic is cleanliness. This typically refers to presence of lubricants like oil and grease, but may also include rust or scale, which is where things get complicated. Corrosion is a form of oxide and it’s almost impossible to have an oxide-free surface. Stainless and steel and aluminum are two examples of rapid oxide layer formation.
6 reasons for quantifying surface quality
- Friction
When two surfaces are brought into contact, high points – asperities in the language of surface finish – grind together, creating friction. If however, rather than mountain peaks the asperities are flat-topped, the friction will be much lower.
Friction also applies to fluids that pass over a surface: a rougher surface will retain a thicker boundary layer and reduce the rate of flow.
- Wear rate
Surface roughness also influences the rate at which a surface wears. Small, sharp asperities erode faster than larger, flat areas, and can quickly lead to undersized parts.
- Adhesion
Bonding mechanisms are complex and not fully understood. However, surface roughness does play a role. Generally speaking, a rougher surface provides a better “key” for a coating to “lock” into, up to a point: get too rough and bond strength stops increasing.
- Appearance
Surface roughness and texture affect how light reflects. A rougher surface provides more scattering and the direction of any texture alters how a surface looks when viewed from different directions.
- Corrosion resistance
A pitted surface provides more locations where corrosion can take hold.
- Biological contamination
As with corrosion, deep surface pitting leaves regions that may not receive adequate sanitization.
Specify the finish needed
The surface of a metal part can determine how it functions in service. When itsquality isn’t defined the result is often sub-optimal performance and perhaps also reduced life. Take time to consider the behaviors needed, specify the surface appropriately, and produce a better product!
